1. Field of the Invention
The present invention relates to heaters in ceramics, and relates in particular to heaters in ceramic susceptors employed in CVD devices, plasma CVD devices, etching devices, and plasma etching devices for manufacturing semiconductors, and in liquid-crystal manufacturing apparatuses.
2. Background Art
In order for the film-formation rates or etching rates in CVD (chemical vapor deposition), plasma CVD, etching, or plasma etching on a semiconductor wafer retained in a film-deposition chamber to take place uniformly, the wafer surface temperature must be strictly controlled. For the purpose of such temperature control, a heater is built into a wafer-retaining member, the surface of the wafer-retaining member is heated, and a wafer of semiconductor material is heated by thermal transfer. Ceramics endowed with heat-resistant, corrosion resistant and insulative properties, such as aluminum nitride and silicon nitride, have been employed to date as wafer retaining members of this sort.
A wafer retaining member made of ceramic into which the foregoing heater is built then has been manufactured according to a method that amounts to sintering aluminum nitride and building-in a molybdenum coil, by training a molybdenum coil into a groove formed in for example a disk-shaped aluminum nitride plate, sandwiching it with another such aluminum nitride plate, and hot-press sintering the sandwich.
In a wafer retaining member made of ceramic into which a heater is built, i.e., a ceramic susceptor, the constituent components of the heater resistive heating element are regarded as elemental impurities, even in trace amounts, with respect to a semiconductor material for silicon wafers or the like, or a liquid crystal material, and can become the source of malfunctioning in semiconductor chips and liquid crystals.
Given the impurity threat, either a resistive heating element must be completely embedded into a ceramic susceptor so as not to appear on the surface, or else a resistive heating element formed superficially on a ceramic must be coated with a protective layer, within the chamber of semiconductor manufacturing apparatuses. Consequently, an area in which the heating element is not buried, i.e., a non-heating area, will necessarily be present on the outer peripheral portion of the ceramic susceptor. The heat generated by the resistive heating element is transmitted through the ceramic, reaching the surface, and from the surface then radiates or escapes via gases due to heat transfer. This means that in disk-shaped or rectangular plate-shaped ceramic susceptors the outer peripheral margin is therefore the place where heat is most liable to escape.
Owing to the above-noted two causative factors taken together, the outer periphery of a ceramic susceptor is the portion where temperature is most prone to drop. To address this issue, elimination of difference in temperature by using for the ceramic a material whose thermal conductivity is high, to swiftly diffuse toward the outer periphery the heat generated by the resistive heating element, has been practiced. Likewise another expedient has been to try to eliminate the temperature difference by increasing the winding density of the coil and the pattern density of the resistive heating element the more toward the outer periphery of the resistive heating element they are, to raise the heating density along, compensating with heat in, the outer periphery.
When a coil trained into a groove in a molded ceramic body is sandwiched between molded ceramic bodies and worked in a hot press, however, it becomes squashed into an indefinite shape and the outer peripheral edge of the resistive heating element in its substantive domain becomes disrupted. The consequence of this has been that despite a resistive heating element being isothermally designed by strictly reckoning how much heat it puts forth and compensation for heat dispersion to its non-heated portions and for heat escape from its edge portion, in practice, the substantive heat-issuing domain becomes disrupted in the edge portion, which has made it impossible to obtain desired isothermal rating in the surface entirety of the ceramic susceptor.
Meanwhile, with the scaling-up of semiconductor wafer size in recent years, isothermal demands on ceramic susceptors for heating the wafers have become stricter, with an isothermal rating in the wafer-retaining face of at minimum within ±1.0%, preferably within ±0.5% being required.